1. Field of the Invention
The present invention relates to a clutch construction in an automatic transmission for a vehicle.
2. Description of the Prior Art
In an automatic transmission mounted in an automobile etc, a planetary gear mechanism and engaging elements such as clutches are combined, and a plurality of gear positions are realized by a combination of engaging/disengaging of engaging elements.
Such an automatic transmission clutch construction is disclosed in for example Laid-open Japanese Patent publication number 6-185613.
In this clutch 80, as shown in FIG. 2, a clutch drum 81 is a one-piece molding produced by press forming, and a spline 83 is formed on a tubular outer drum 82 thereof.
A plurality of friction plates 93 that respectively mesh with a spline 91 that is formed on a clutch hub 90 arranged facing the inside of the outer drum 82 and the aforesaid spline 83 are arranged alternately.
Within the clutch drum 81, there is provided a piston 88 for applying pressure to the friction plates 93. A pressure chamber 89 for driving this piston 88 is formed between this piston and the clutch drum 81. In order to achieve an oil-tight stroke of the piston 88, the outer circumferential surface of an inner tube 84 of the clutch drum 81 is made to serve as a sliding surface on the radially inwards side of the piston and the inner circumferential wall of a projection 86 formed on a vertical wall 85 is made to serve as a sliding surface on the radially outwards side of the piston.
However, the projection 86 forming the aforesaid piston sliding surface projects to the outside from the vertical wall 85 rather than to the interior of the clutch drum that accommodates the friction plates 93 and the clutch hub 90. Furthermore, since the entire clutch drum 81 is constituted as a one-piece press molding, radiussing (R) is necessarily required at the corner portion in the cross-section, so the length dimension S of the projection 86 must take into account this radiussing in order to ensure the prescribed piston stroke.
Therefore the overall length of clutch drum 81 in the axial direction has to be large.
An object of the present invention is therefore to provide a clutch construction for an automatic transmission wherein the surroundings of the piston of clutch drum are made compact, enabling the axial length to be shortened.
To this end, the clutch construction according to the present invention comprises:
a clutch drum composed of an inner drum comprising a hub part, disc part and outer tube, and an outer drum having a drum and a vertical wall joined to the radially outside of the inner drum; a clutch hub arranged facing the outer drum within the clutch drum; a plurality of friction plates that alternately engage with the outer drum and the clutch hub; and a piston for applying pressure to the friction plates; wherein said inner drum constitutes a cylinder that accommodates the piston and the outer circumferential surface of the hub part and inner circumferential surface of the outer tube form sliding surfaces for the piston.
Since the outer tube of the inner drum is formed in a straight line in the axial direction as far as its tip, its inner circumferential surface as far as its tip can be utilized as a piston sliding surface of sufficient length even though the outer tube is short. In this way, the benefit is achieved that the necessary sliding surface length can be ensured even though the length of the clutch drum in the axial direction is shortened.
By providing the inner drum with a step at the corner from the disc part to the outer tube, the vertical wall of the outer drum being butt welded to this step, joining of the inner drum and outer drum can be achieved with high precision in the axial direction and radial direction.
Furthermore, by arranging the friction plates outside of the outer tube and the piston having an extension that extends outwards beyond the outer tube and comprising a portion that applies pressure to the friction plates at its radially outer end, this extension being offset towards the vertical wall of the outer drum after crossing the tip of the outer tube, the length of the clutch as a whole in the axial direction can be further shortened whilst guaranteeing a sufficient stroke gap with respect to the clutch hub.
A centrifugal canceling piston may be further provided on the outer circumferential surface of the hub part of the inner drum, this centrifugal canceling piston having an external diameter corresponding to the inner circumferential surface of the outer tube. The pressure-receiving area of each piston can thereby be equalized simply by altering the diameter of the outer tube; thus piston control with excellent balance can be obtained.